Recently, some flavonoids have been found to have antitumoral activity (Verna, Cancer Research 48, 5754, 1988) and chemoprophylactic activity in some tumors (Cassady, J. Nat. Prod. 53, 23, 1990). Particularly quercetin, a flavonoid which is almost ubiquitous in plants, has proved some inhibiting activity on the proliferation of human leukemia cells (Larocca, Br. J. of Haematology 75, 489, 1990) and on other cell lines (Scambia, Br. J. Cancer 62, 942, 1990--Int. J. Cancer 46, 1112, 1990--Cancer Chemother. Pharmacol. 28, 255, 1991--Gynecologic Oncology 45, 13, 1992) besides having a synergistic activity with the usual chemotherapeuticals. Though the mechanism of such an inhibiting action on proliferation is unknown, it seems to be connected with the interaction of this flavonoid with the estrogen receptors of type II (Markaverich, J. steroid Biochem. 30, 71, 1988). These receptors, first described by Clark (J. Biol. Chem. 253, 7630, 1978) in the rat uterus, are different from the real estrogen receptors (ER) since these are present in a higher concentration and have a dissociation affinity constant (K.sub.D : 10-20 nM) for estradiol lower than that of the estrogen receptors (K.sub.D : 0.2-1 nM).